In recent years, cognitive radio systems that detect and use an unused frequency band have been studied and developed. Usable frequency bands change with time and location due to communications by primary users, the mobility of terminals, or other reasons. Therefore, it is important to determine an appropriate frequency band to be used for communication quickly.
In many studies on cognitive radio systems, it is assumed that there is a common control channel.
For example, non-patent literature 1 discloses a method that utilizes a common spectrum coordination channel (CSCC). Each radio node broadcasts frequency usage information (e.g. user ID such as IEEE MAC address, frequency band being used, and transmission power etc.) in the CSCC at regular time intervals. Neighboring nodes observe such notifications, and a user that has newly become active creates a spectrum activity map, and if there is a usable frequency, the user selects that frequency.
Non-patent literature 2 describes utilizing a central spectrum policy server (SPS) in order to coordinate spectrum requests made by a plurality of cognitive radio network operators. In the technique according to this study, spectrum allocation is performed by a center in a centralized manner, and it is assumed that a common control channel is used for spectrum allocation and spectrum coordination.
In a technique described in non-patent literature 3, each radio node participates in a local coordination group based on similarity in the usable channel to form a small multi-hop network. In this group, interconnections are maintained by using a common control channel, and the members in the same group communicate directly with each other. The network connection is maintained by users that are participating in a plurality of coordination channels and located at group boundaries. Such “bridge” nodes relay traffics between groups and interconnect users in spatial domains of different spectrum availability.
In systems, such as IEEE 802.22 systems, in which there is a base station (BS), the BS allocates frequency channels to terminals based on the results of sensing of the frequency use status by the terminals. Since the allocation of frequency channels is performed by a control section such as a BS, it is not necessary for transmitting node and receiving node to notify each other of a frequency channel to be used when establishing a connection. In addition, if the radio condition changes during communication, and the frequency channel being used is to be changed, the frequency channel can be easily changed by a command from the BS.